Convergence of three mangrove species towards freeze‐tolerant phenotypes at an expanding range edge

Abstract

Summary Climate change is dramatically altering the distribution and abundance of many species. An examination of traits may elucidate why some species respond more strongly to climate change than others, particularly when ecophysiological thresholds set range limits. Mangrove forests are expanding polewards. Although multiple environmental factors influence mangrove distributions, freeze tolerance is hypothesized to determine their poleward extent. To investigate how trait variation influences mangroves’ responses to a warming climate, we examined how freeze tolerance and associated traits varied along a latitudinal cline for three co‐occurring mangrove species. We sampled individuals along >200 km of Florida, USA's eastern coast, from the mangroves’ most northern populations, where freeze events were historically common, to southern populations where freeze events continue to be rare. We measured a suite of traits in field‐collected adults and their garden‐reared offspring, and assessed their responses to an experimentally imposed freeze event. We asked whether freeze tolerance and other traits varied predictably among species, with latitude, and between life stages. Species and populations varied dramatically in freeze tolerance, with the highest freeze tolerance in the northernmost species and populations, and the lowest freeze tolerance in the southernmost species and populations. Additionally, leaves of all three species were drier, tougher, thicker and more freeze‐tolerant at the range edge. Tolerance to freezing appears to set the range limits for these mangrove species. All three species converged on a similar phenotype at the range edge, but species‐level variation in freezing resistance was conserved. Thus, these species are likely to continue migrating at different rates in response to climate warming, potentially leading to the dissolution of typically co‐occurring species and creating ‘no analogue’ coastal mangrove–marsh communities.